What is State of Health (SoH)? Everything about your EV battery's condition

State of Health, or SoH, is a single percentage that describes how much usable energy capacity an EV battery still has left, compared to when it rolled off the factory line at 100%. A battery reported at 90% SoH can still store and deliver roughly 90% of the energy it could when new — the chemistry hasn't failed, it has simply aged, the same way a phone battery lasts less than a full day after a few years of daily charging.

For anyone buying a used EV, SoH is the closest thing to an odometer reading for the single most expensive part of the car. This guide covers what the number means, how it differs from the percentage on your dashboard, how it's actually measured, what counts as a good or concerning reading, and why a professionally measured report beats guessing from age and mileage alone.

State of Health: the health gauge of your battery

Think of a new EV battery as a glass bottle that holds exactly one litre. Over years of being filled and emptied, the glass doesn't shatter or leak — but the bottle itself very gradually shrinks, so eventually it only holds 900ml. SoH is that shrinkage expressed as a percentage of the original size: a bottle holding 900ml out of its original 1000ml is at 90% SoH. Nothing is broken; the battery has simply aged, and it now has less to give on every full charge.

SoH versus SoC: two very different things

SoH is very commonly confused with State of Charge (SoC) — the percentage shown when you plug in and charge the car. They answer completely different questions.

A car showing "100%" charged with an 85% SoH battery is completely full — of a smaller usable capacity than when the car was new. Charging to 100% doesn't tell you anything about SoH; it just fills whatever capacity is currently available.

How is SoH measured?

There are three broad approaches in use today, each with a different balance of accuracy, cost, and convenience.

1. Capacity-based measurement

The most direct method: charge the battery fully, then discharge it under controlled conditions while measuring exactly how much energy comes out. Comparing that figure to the battery's original rated capacity gives a highly accurate SoH reading — but it requires specialised equipment and takes time, since a full discharge cycle can't be rushed without skewing the result.

2. Impedance measurement (internal resistance)

As a battery ages, its internal resistance rises in a fairly predictable way. Measuring that resistance and comparing it against known ageing curves for the specific battery chemistry gives a faster, non-destructive estimate of SoH — useful as a cross-check, though generally less precise on its own than a full capacity test.

3. OBD-II readout via the BMS

Every EV has a Battery Management System (BMS) that continuously tracks the battery's condition internally. Reading that data via the car's OBD-II diagnostic port is fast and requires no physical test of the battery itself — but it reports whatever the manufacturer's own BMS software has calculated, which can be conservative, optimistic, or simply calibrated differently depending on the brand, and isn't independently verified.

In practice, the most reliable readings combine more than one of these approaches rather than relying on a single method — which is exactly why a proper independent report is worth more than any single number pulled from one source.

What counts as a good SoH value?

There's no single pass/fail line — SoH exists on a spectrum, and what's "good enough" depends partly on how you plan to use the car. As a general benchmark for a used EV:

These bands are the same six tiers used throughout every Battery Truth report, so a verdict you see on a report always maps back to this same scale.

When should you start to worry?

• The car's real-world range has dropped noticeably more than its age and mileage would suggest, compared to similar examples.
• SoH is reported below roughly 80% well before the manufacturer's stated warranty threshold or distance/age limit.
• The battery has a known history of frequent deep discharges (regularly down to near 0%) or long periods parked at 100% in hot weather.
• The car was previously used for ride-hailing, delivery, or another high-utilisation role with heavy fast-charging reliance.

Can you check your EV's SoH yourself?

A handful of consumer apps can read basic battery data from many EVs via the OBD-II port or the manufacturer's own companion app, giving a rough sense of battery condition. They're a reasonable first-pass sanity check, but they come with real limitations worth knowing before you rely on one to make a purchase decision.

What do manufacturers guarantee?

Most manufacturers back their EV batteries with a warranty against dropping below a stated SoH threshold within a set number of years or distance, whichever comes first — a useful signal of what the industry itself considers normal ageing versus a defect. Typical terms look like this:

• Tesla — battery and drive unit warranty, typically 8 years, with a minimum retained capacity threshold that varies by model.
• Nissan — battery capacity warranty, typically 8 years / 160,000 km, guaranteeing against dropping below a set number of remaining capacity bars.
• Hyundai / Kia — battery warranty, typically 8 years / 160,000 km, against falling below roughly 70% SoH.
• BMW — high-voltage battery warranty, typically 8 years / 160,000 km, with a stated minimum capacity threshold.
• Volkswagen Group (VW / Audi / Škoda) — battery warranty, typically 8 years / 160,000 km, against falling below roughly 70% SoH.

Warranty terms vary by market and model year, so always check the specific figures for the exact car in front of you — but the pattern across the industry is consistent: manufacturers themselves treat SoH loss down to roughly 70-80% within 8 years as expected, normal ageing rather than a fault.

Want to know your EV's actual SoH?